Analytical instrumentation refers to a broad variety of instruments that provide information on the composition of matter. Analytical instruments are important in many manufacturing and industrial processes, such as to assess/verify the purity of raw materials, to identify the progress or completion of chemical reactions, to determine defined stages in refining or other chemical transformation processes, to sort manufactured products by chemical purity, for quality control, and the like.
One type of analytical instrument that has application in a broad array of manufacturing and industrial processes is a Raman spectrometer. Raman spectroscopy is a method of ascertaining and verifying the molecular structures of materials. Raman spectroscopy relies on inelastic scattering, or Raman scattering, of monochromatic light, resulting in an energy shift in a portion of the photons scattered by a sample. From the shifted energy of the Raman scattered photons, vibrational modes characteristic to a specific molecular structure can be ascertained. In addition, by analytically assessing the relative intensity of Raman scattered photons, the concentration of a sample can be quantitatively determined.
The Raman effect occurs when light impinges upon a molecule and interacts with the electron cloud and the bonds of that molecule. For the spontaneous Raman effect, which is a form of light scattering, a photon excites the molecule from its ground state to a virtual energy state. The energy state is referred to as virtual since it is temporary, and not a discrete (real) energy state. When the molecule relaxes, it emits a photon and it returns to a different rotational or vibrational state. The difference in energy between the original state and this new state leads to a shift in the emitted photon's frequency away from the excitation wavelength.
If the final vibrational state of the molecule is more energetic than the initial state, then the emitted photon will be shifted to a lower frequency in order for the total energy of the system to remain balanced. This shift in frequency is known as a Stokes shift. If the final vibrational state is less energetic than the initial state, then the emitted photon will be shifted to a higher frequency, which is known as an Anti-Stokes shift. Raman scattering is an example of inelastic scattering because of the energy transfer between the photons and the molecules during their interaction.
The pattern of shifted frequencies is determined by the rotational and vibrational states of the sample, which are characteristic of the molecules. The chemical makeup of a sample may thus be determined by an analysis of the Raman scattering. In Raman spectroscopy, a sample is typically illuminated with a laser beam. Light from the illuminated spot is collected by lenses and analyzed. Wavelengths close to the laser line due to elastic Rayleigh scattering are blocked or filtered out, while chosen bands of the collected light are directed onto a detector. The spectra of these photons are analyzed to identify peaks resulting from concentrations of Stokes and Anti-Stokes shifted photons. The spectra are characteristic of the molecular structure of the sample, and the amplitude of the peaks may be analyzed to ascertain relative concentrations of identified molecules in the sample. Of course, Raman spectroscopy is just one of many types of analytical instrumentation useful in many manufacturing and industrial processes.
The environments in which many manufacturing and industrial processes take place are not conducive to, or present a hazard to, any type of electrical or electronic equipment, including analytical instrumentation. For example, many industrial environments present a fire hazard. The US National Fire Protection Association publishes NFPA code 70, also known as the National Electrical Code (NEC). The NEC defines three classes of fire hazardous conditions based on the type of fire hazard present: Class I (gas and vapor), Class II (dust), and Class III (fibers and flyings). Each Class is divided into Division 1 (hazardous condition normally present) and Division 2 (hazardous condition not normally present but may accidentally exist). The Classes are further subdivided into groups based on the specific material giving rise to the fire hazard.
Conventionally, electronic equipment operative in a NEC Class I Division 1 (C1D1) environment—for example, a petroleum refinery where flammable or explosive gases or vapors are normally present—is protected by actively purging oxygen from the equipment and replacing it with an inert gas. The equipment is located in a housing, and all air within the housing must be purged using an inert gas such as nitrogen a predetermined number of times (e.g., thrice), and then maintained with a dynamic positive pressure of the inert gas, relative to the surrounding atmosphere, prior to any electronics being activated. This ensures that the interior of the housing is non-incendive, and the electronics cannot cause a fire in the event of a spark, arc, overheating, or the like. The purge requires an external source of inert gas, such as a tank, and associated gas conduction lines, valves, pressure sensors, a controller, and the like. The dynamic, positive inert gas pressure is generally maintained by continued connection to the external inert gas tank, overcoming small leaks from the housing by applying a constant pressure of inert gas to the equipment. Such purging and dynamic pressure maintenance equipment is bulky, inconvenient, and expensive, and the purging process introduces delay in utilizing the electronic equipment.
Aside from fire hazard, many analytical instrumentals must operate in inhospitable environments, such as wet locations, or locations in which they are exposed to chemical drips, mists, or vapors. Protection of the electronics or optical systems in these instruments from such environmental hazards is important.
Analytical instruments are often deployed to monitor processes or production environments that are tightly regulated, such as pharmaceutical manufacturing. In such environments, it is valuable to be able to prove that an analytical instrument has not been tampered with, or modified to alter the reported results of a measurement. Additionally, the integrity of analytical equipment housings may be valuable, such as to verify that chemical contamination has not occurred, or to access the validity of warranty claims. A variety of tamper-proof and tamper-evident equipment housings and seals are known in the art. However, these functions increase the cost of the equipment, and may require special tools, chemical detectors, codes, and the like to effect the tamper-proof or tamper-evident function, which additionally add cost and complexity.
A particular concern with analytical instruments utilizing lasers, such as Raman spectrometers, is compliance with laser safety regulations. The American National Standard Institute publishes ANSI Z136 defining classes of lasers based on power and wavelength, and prescribing associated required safety measures, such as labeling and the use of safety goggles. Where high power lasers are employed, the laser source and optical paths may be carefully positioned and maintained to prevent or minimize interaction with the beam path, to mitigate the risk of eye injury or other laser hazard. Since access to the laser source could alter the laser beam path in a way that may result in violation of the applicable standards, analytical instruments utilizing powerful lasers typically include mechanical switches on the equipment housing, coupled to interlock circuits that cut off power to the laser source if the housing is opened. Both the switches and interlock circuitry add cost and complexity to the instrument.
Analytical instruments often include sensitive detectors or transducers, such as Charge Coupled Devices (CCD) in optical instruments, which are cooled to improve their sensitivity. For example, operation at lower temperature reduces the rate of natural thermal electron-hole formation in semiconductor materials. Thermal hole-electron pairs increase the shot noise and reduce the ultimate sensitivity of the detector. One hazard to cooling electronic circuits or subsystems is the condensation of water from warmer, ambient air. Condensation can damage a detector, cause short circuits in electronics, promote rust and corrosion, and the like. To combat condensation, cooled electronics conventionally must provide dehumidification, such as by purging the system with a dry gas, similar to the inert gas purge discussed above for fire hazard protection. Such dehumidification systems add bulk, cost, and complexity to analytical instruments.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.